1. Field of the Invention
The present invention relates to a spark plug, and more particularly to a spark plug for use, for example, in an internal combustion engine.
2. Description of the Related Art
A spark plug used in an internal combustion engine, such as an automobile engine, generally includes a center electrode; an insulator which holds the center electrode; a metallic shell which holds the insulator; and a ground electrode whose one end is joined to a front end portion of the metallic shell and whose other end faces an end portion of the center electrode so as to form a spark discharge gap therebetween. The metallic shell has a mounting threaded portion at its outer circumference, for attachment to an engine or the like.
A front end portion of the insulator is inserted into the metallic shell of the spark plug from the rear end of the metallic shell toward the front end of the metallic shell. Subsequently, a rear end opening portion of the metallic shell is crimped so as to fix the metallic shell to the insulator. A packing is interposed between a step portion provided on the outer surface of the insulator and a step portion provided on the inner surface of the metallic shell, and an annular space between an outer surface of the insulator and an inner surface of the metallic shell is filled with a powder composed mainly of talc, thereby preventing gas leakage from a combustion chamber of the internal combustion engine.
3. Problems to be Solved by the Invention
In order to prevent gas leakage from a combustion chamber of an internal combustion chamber, when the metallic shell is to be crimped to the insulator, a sufficient crimping load must be imposed for ensuring gas-tightness of the junction between the metallic shell and the insulator. However, a large crimping load is not absolutely acceptable. Namely, an excessively large crimping load deforms a trunk portion of the outer surface of the metallic shell in a region contacting a crimping die. In the case of a spark plug in which a portion of the metallic shell contacting the crimping die assumes the form of an annular plane, and a gasket is disposed on the annular plane portion for ensuring gas-tightness at a peripheral region of an opening of a mounting hole of an internal combustion engine, the deformation rate in relation to the crimping load is low. Thus, the above problem does not arise often. However, when a portion of the metallic shell contacting the crimping die assumes a taper form, an excessive crimping load may deform the taper portion of the metallic shell.
Recently, in association with improved engine control technology and an increase in the number of valves, an increasing number of components are disposed around the engine. Accordingly, a volume allocated for a spark plug is decreasing, so that a reduction in spark plug size is eagerly desired.
A reduction in spark plug size is accompanied by a reduction in the area of the packing provided for preventing gas leakage from a combustion chamber of an internal combustion engine and a reduction in the volume of the annular space into which talc is filled. Thus, in order to ensure gas-tightness, the crimping load must be increased. Meanwhile, since a reduction in the size of a spark plug is accompanied by a reduction in the size of the metallic shell, imposition of a large crimping load makes the taper portion of the metallic shell more susceptible to deformation.